In one kind of liquid metal cooled nuclear reactor which is a fast breeder reactor the fuel assembly is supported on a diagrid and submerged in a pool of liquid metal in a vessel. The diagrid is carried on a support structure from the wall of the vessel and serves as a plenum for distributing coolant throughout the fuel assembly. The fuel assembly comprises a plurality of replaceable sub-assemblies which upstand from the diagrid in side-by-side array and are arranged in groups. Each group of sub-assemblies is carried on a carrier of massive steel which serves to protect the diagrid against damage during fuel replacements and by irradiation. In our U.S. Pat. No. 4,256,538 for liquid metal cooled fast breeder nuclear reactors there is disclosed a construction of this kind of liquid metal cooled fast breeder nuclear reactor wherein the diagrid comprises a discrete plenum and an undershell. The undershell normally carries the entire load of the fuel assembly thereby leaving the plenum substantially load free, the loading being freely transmitted through the plenum by means of a plurality of struts formed integrally of the carriers. In the event of failure of the undershell the normally unloaded plenum can accept the load of the fuel assembly and in the event of a breach of the plenum the undershell, in cooperation with other reactor structure, provides a partial secondary containment whereby the rate of leakage from the breach is reduced sufficiently to maintain the reactor safe pending shut down. However, in order to reduce the capital cost of the construction it has been proposed to eliminate the carriers and in published British application No. GB 2,051,459A there is described a construction wherein the sub-assembly is supported on a discrete resilient spike upstanding through the plenum from the lower face plate, shielding of the diagrid being achieved by massive steel bushes interposed between the sub-assemblies and the upper face plate of the diagrid. The spikes, which are readily replaceable when damaged during refuelling operations, provide a measure of flexibility whereby the sub-assemblies can be displaced laterally to avoid serious stressing in a peripherally restrained fuel assembly. In this construction, of course, the functions of plenum and fuel assembly support are not separated and there is no inherent safeguard of the coolant flow in the event of failure of the plenum.
It is an object of the present invention to provide a construction of liquid metal cooled nuclear reactor not including carriers for the fuel sub-assemblies but wherein the functions of fuel assembly support and coolant plenum are performed by discrete components of the diagrid each of which can serve the function of the other in the event of failure of one of the components.